An explosion that injured two men Sunday night, marking the fourth blast in Austin, Texas this month, could've been triggered by a tripwire, Austin police Chief Brian Manley said.

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Authorities are working under the belief that the latest incident is connected to the previous three explosions in the city, Manley said. At this point, information is preliminary, and police have yet to fully process the scene.

Manley said it's "very possible" that the device was activated by someone either handling, kicking or coming into contact with a tripwire that activated the device.

Latest developments

The package on Sunday was left on the side of the road. The two injured men were either biking or pushing bicycles when the explosion occurred. Police are waiting for daylight to process the scene. Residents living near the scene have to stay indoors until 10 a.m. Hours before the latest blast, the chief appealed directly to whoever was responsible for the bombs.

Chief advises further caution

The two men, in their 20s, were taken to the hospital with serious but non-life threatening injuries, authorities said. Both patients are in good condition, a spokesperson at St. David's South Austin Medical Center told CNN.

Police and the FBI responded to the scene Sunday. There was a second item, a backpack that police were clearing, said Austin Police Chief Brian Manley.

The victims in those three explosions were all African-American or Hispanic. Police have not yet discovered a motive, but have not ruled out the possibility the bombs could be hate crimes.

"Do not touch any packages or anything that looks like a package. Do not even go near it at this time," Manley told residents. "Given the darkness we have not had an opportunity to really look at this blast site to determine what has happened."

On Monday morning, he added to that warning: "We now need the community to have extra level of vigilance and pay attention to any suspicious device -- whether it be a package or a bag, a backpack -- anything that looks out of place. And do not approach items like that."

Resident: 'It's concerning'

Stan Malachowski, who lives about half a mile away, said he heard a loud explosion.

"It was loud enough to hear inside of our house with our windows and door shut. Again, airplanes go by and cars backfire so we didn't think much of it," he told CNN affiliate KXAN.

"This is a quiet neighborhood. It's a family neighborhood. It's concerning."

The Sunday incident was reported on Dawn Song Drive, with police warning residents in the immediate area to stay inside their homes until morning. Regents School of Austin, a nearby private school, will open two hours late Monday for "a complete security sweep," it said in a statement.

The Austin Independent School District announced that it would not be able to send school buses into the affected Travis Country neighborhood Monday, because of police activity. "Any tardies or absences due to this situation will be excused," the district posted on Facebook.

On edge

Many in Austin have been on edge since the bombings, as some residents of color in the Texas capital say they feel under threat.

The reward for information leading to the arrest of the person or persons responsible for the three explosions increased to a total of $115,000, authorities had announced earlier Sunday. Hours before the most recent explosion, Manley appealed directly to the person or persons responsible.

"We believe that the recent explosive incidents that have occurred in the city of Austin were meant to send a message," Manley had said in a press conference.

"We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed out of this event," he added.

Officials have urged residents to call in with tips to the police department, even if the information is seemingly "inconsequential."

South by Southwest wrapped up Sunday, but received a bomb threat Saturday that resulted in the cancellation of a concert featuring The Roots.